Expandable polymeric bead or pellet material has been employed to prepare a variety of foam plastic products. In particular, solid, expandable, polystyrene pellets have been expanded for use in closed or open molds to form coffee cups and other molded foam articles, and also for use in preparing free-flowing, highly expanded foam pellets of a variety of shapes for cushioning and packaging purposes.
The expandable polymeric pellets typically contain in their preparation a blowing agent, so that, when the pellets are exposed to heat, typically by the application of steam, the pellets expand to many times their size; that is, ten to fifty times. For example, expandable, polystyrene, solid pellets are used to prepare free-flowing, expandable, polystyrene foam packaging. Such expandable pellets are expanded by the application of heat through the use of steam in a tumbling drum. The pellets are expanded while tumbling or being agitated in the expansion phase, in order to prevent fusing together of the individual pellets.
One device employed to expand polystyrene pellets is described, for example, in U.S. Pat. No. 3,751,218, issued Aug. 7, 1973, hereby incorporated by reference. This device employs a plurality of paddles on thin rods extending from the internal surface of a horizontal revolving drum, to provide for a tumbling of the pellets and moving of the pellets from the inlet to the outlet of the drum, while the pellets are subjected to steam to effect the expansion of the pellets. The expandable polystyrene pellets may be fed by a helical-screw or other metering feed pump to the drum inlet and the pellets permitted to fall by gravity into the interior of the drum. Usually, the expandable pellets are solid and have a density of from about 35 to 50 pounds per cubic foot at the inlet and are expanded in one or more passes through the interior of the drum, usually in a time period of from 1 to 10 minutes; for example, 1 to 3 minutes, to a density of from about 1.0 to 1.5 pounds per cubic foot employing a temperature of from about 96.degree. C. to 99.degree. C. The expanded foam pellets of the first pass are usually subjected to a second pass through the steam-expanding tumbling apparatus, in order to reduce the density of the foam pellets further to about 0.5 to 0.6 or less, with a time period generally of less than about 3 minutes in the second pass. Often, it is desirable to provide an additional pass through the expanding device, to reduce the foam density to the lowest level possible; for example, as low as 0.3 pounds per cubic foot, with a residence time of from 1 to 2 minutes.
The expandable polystyrene pellets being manufactured may contain, as part of the expandable composition, a small, but effective, amount of a lubricant, such as, for example, ten to three hundred parts per million, in order to permit the expandable pellets to tumble freely; that is, to provide for a free-flowing, nonfusing, tumbling action by the pellets during the expansion process. In some cases, the lubricant is added directly to the solid expandable pellets after manufacture, such as by blending the lubricant with the pellet polymer composition, or just before feeding the pellets into the steam-expanding tumbling drum. A typical lubricant employed, either in the composition or used to coat the exterior surface of the solid, expandable, polystyrene pellets, would be a metal soap, such as a calcium or zinc stearate.
In some cases, a lubricant is not employed; however, the omission of a coating lubricant often reduces the production flow rate of expansion. For example, in an expansion device of the type described, the use of a lubricant permits production rates of over 500 to 700 pounds per hour, while the omission of a lubricant often reduces the rate to 300 to 500 pounds per hour and introduces pellet-fusion problems. Thus, a lubricant, while adding to the cost, provides for a better tumbling action, but the use of a lubricant causes some difficulty in the use of the foam pellet material.
The loose, expandable polystyrene pellets used, for example, for packaging material, comprise a thermoplastic polymer, such as polystyrene, which has a chemical or mechanical blowing agent incorporated therein and in an amount sufficient to effect the desired expansion. Typical blowing agents include those agents which decompose and provide an inert expandable gas, or those liquid volatile agents which expand or are volatilized on the application of heat, such as low-molecular-weight hydrocarbon liquid, such as isopentane, butane, pentane, or fluorocarbons or any combination thereof.
One of the problems associated with the use of metal stearates, such as zinc stearate, as a polystyrene pellet lubricant is the effect of the lubricant on the free-flowing foam pellets for packaging. The metal stearate is employed as a lubricant to provide a good tumbling action in the expansion process of the expandable pellets. However, the effect of the lubricant on the expanded free-flowing foam particles and the packaging or cushioning of articles therein is to permit the outside of the expandable foam pellets to be of higher lubricity than is desired. Typically, for example, when zinc stearate is employed as a lubricant which is dusted over expandable pellets, the resulting free-flowing foam pellets, used in the packaging material, cause the package to move downwardly in the foam mass, due to the slipperiness of the foam pellets. The package then tends to move downwardly in the exterior container and to rest on the bottom of the container in which the article to be packaged is placed. This disadvantage is particularly evident with heavier objects which are packaged in the free-flowing foam pellets. When the article moves to or near the bottom of the packaging container or to the side, the article is more likely to be damaged during transportation, due to the lack of the surrounding foam cushion and insulation of the foam pellets.